Low cardiac output syndrome is a common clinical problem early after mitral valve replacement. In addition, these patients often have increased pulmonary vascular resistance (PVR) which may be exacerbated by commonly used inotropic agents. This study compared the hemodynamic effects of dopamine and dobutamine, over a range of therapeutic doses, in 10 patients within several hours after mitral valve replacement. Both dopamine and dobutamine are effective inotropic agents, producing progressively increasing cardiac output and arterial pressure with increasing drug dosages. Both agents also cause moderate tachycardia and we did not demonstrate any difference in this effect between the two drugs. While dobutamine had no effect on PVR, there was a significant decrease in PVR with dopamine infusion.